The field of the invention pertains to injection molding of foamable plastic products and, in particular, to the provision of a smooth surface finish on the foamed plastic products.
For aesthetic and functional reasons a smooth, shiny and blemish free surface finish is desired on many foamed plastic products. Such a smooth and blemish free surface finish can be provided by molding with dies heated to a relatively high temperature (300.degree. F.-550.degree. F.) prior to injecting the foamable plastic into the die cavity. The upper temperature is typically selected to be above the heat distortion temperature of the foamable plastic. The dies must then be cooled to approximately 140.degree. F. before the mold is opened and the product ejected. The repeated heating and cooling of the relatively massive bulk of the mold requires considerable time and energy.
In order to substantially lessen the cycle time necessary to heat and cool the mold, means for heating only the cavity surface just prior to injection of the foamable plastic have been developed. Exemplifying this approach in the prior art is U.S. Pat. No. 4,201,742. Disclosed is a method for heating the mold cavity surface by means of condensing steam thereon and draining the condensate from the cavity just prior to injection of the foamable plastic resin. This method provides a smooth surface finish on the product as desired, however, despite much effort to overcome water marks and streaks caused by condensate near the bottom of the mold cavity, such defects continue to be a problem.
A method and apparatus for dry heating the cavity surface is shown in U.S. Pat. No. 2,979,773 wherein a semiconductive film is coated on the mold surface and an aluminum electrode plated onto the semiconductive film. The mold body constitutes the other electrode. A heating current passes through the entire mold with the greatest temperature rise occurring in the semiconductive film. Such a construction is limited to relatively short production runs because the aluminum and semiconductive film coatings are subject to rapid wear with repeated molding cycles.
U.S. Pat. No. 3,173,175 discloses a thin metal resistance element on or embedded in a glass or ceramic insulative layer. Glass and ceramic coatings, however, severely limit the mold steels that can be used to support the glass and ceramic coatings and are very difficult to apply only to those areas of the mold surface to be heated.
U.S. Pat. No. 2,984,887 discloses an electrically heated copper or silver coating or preformed cup inserted in a ceramic mold. The mold is heated for a time sufficient to dry the skin on ceramic green ware such that the green ware shrinks away from the mold and can be removed easily without damage. The cup or liner is porous or only covers part of the mold cavity surface to permit the absorbtion of vapors from the drying ceramic mix. The temperatures are limited and the heating time on the order of a half a minute to drive out the moisture without damage to the part.